In the fields of optics and electronics, higher integration density, information of higher density, and image information with higher definition are required increasingly. For this reason, films with finer structures are strongly desired in such fields. Examples of such films include a polymer film with a honeycomb structure, namely, a plurality of micropores at a μm level in a honeycomb arrangement. The film with a fine structure is also desired in the medical field. For example, a film that provides scaffolds for the cell culture is needed (see, for example, Japanese Patent Laid-Open Publication No. 2001-157574). Such film is more suitable for the cell culture than a smooth film by virtue of fine projections and depressions formed on the film. The fine projections and the depressions are formed substantially uniformly on the film.
However, there is an increasing need for films with nonuniform structures. In the field of regenerative medical research, a material for culturing cells that grow into a living organism needs to have a nonuniform structure because the living organism has a nonuniform structure. A porous material is suggested to be used as a material (hereinafter referred to as cell culture substrate) that provides scaffolds for the cell culture. Such porous material is produced from bioabsorbable polymer and formed with a plurality of pores whose porosity and pore size gradually change (see, for example, Japanese Patent Laid-Open Publication No. 2006-068080).
A vessel for the cell culture is suggested (see, for example, U.S. Patent Application Publication No. 2006/0281172 corresponding to Japanese Patent Laid-Open Publication No. 2006-325532). The cell culture vessel is provided with plural areas for culturing cells. Each area is formed with projections and depressions. Each projection is circular in cross section, and the diameters of the circular cross sections vary with the area.
To culture cells in the porous material, an optimum diameter and an optimum depth of the pores depend on types of cells to be cultured. A suitable diameter and a suitable depth of the pores, for supplying oxygen and nutrition to cells and for releasing carbon dioxide and waste from cells effectively and efficiently, vary among cells. For this reason, screening of the cell culture substrates to find suitable dimensions and suitable conditions for the cell culture shortens time for selecting the cell culture substrate and determining the cell culture conditions.
The material disclosed in the Japanese Patent Laid-Open Publication No. 2006-068080 has a mixed area where the cell culture area and an area not used for the cell culture coexist. In addition, the conditions of the material vary in each area of the material. Therefore, the material is not suitable for screening including reproducibility test. On the other hand, in the material disclosed in the U.S. Patent Application Publication No. 2006/0281172, the diameters of the projections vary from one cell culture area to another, but are substantially identical in each cell culture area. However, there may be cases where the area with the projections and depressions suitable for the cell culture may not be found. In other words, although the cells cannot be cultured effectively in any of the above cell culture areas, there is a possibility that such cells can be cultured using a different cell culture substrate having pores whose diameters are somewhere between the minimum diameter and the maximum diameter of the projections of the material disclosed in the U.S. Patent Application Publication No. 2006/0281172. Therefore, such material is not necessarily suitable for screening.